Seasonal Variation in Particulate Flux in an Offshore Area Adjacent to Coastal Upwelling

Abstract

Successive sediment trap deployments were made over a two-year period at a site 360 km west of Cape Mendocino (39.5°N, 128°W). In the final deployment, the shallowest trap was equipped with a sample changer which provided three consecutive samples during the spring and summer of 1981 in order to study possible seasonal variations due to the influence of coastal upwelling. Biogenic flux data point to increasing productivity at the site from mid-March through the end of August, 1981. The ratio of opal flux to carbonate flux increases from spring to a mid-summer maximum. Radiolarians dominate the spring opal flux, whereas diatoms are the major contributors to opal flux in the mid and late summer. Changes in the distributions of three silicoflagellate species suggest colder temperatures occur in mid to late summer. Upwelling at the latitude of our mooring is greatest in midsummer. Although upwelling occurs near the coast, colder water, possibly with higher nutrient levels, can be advected rapidly offshore in the form of surface plumes. Satellite images from April through July, 1981, reveal at least three plumes crossing our area during the period June 21-July 29. These cold water plumes extended from Cape Mendocino to the mooring location. The occurrence of such plume events can explain the flux patterns and changes in other indicators recorded during the spring-summer period by our multiple sampling sediment trap.